An optical parametric oscillator (OPO) is a light source emitting radiation with properties comparable to that of a laser. OPOs are nonlinear devices that split short wavelength pump photons into two longer wavelength photons, namely signal and idler photons. The wavelengths of the signal and idler photons are not independent from each other, but may be tuned in wavelength.
As shown by FIG. 1, an OPO converts an input laser wave (the “pump”) with frequency ωp into two output waves of lower frequency (ωs, ωi) via second-order nonlinear optical interaction. The sum of the frequencies of the output waves is equal to the input wave frequency: ωs+ωi=ωp. For historic reasons, the output wave with the higher frequency ωs is called the signal, and the output wave with the lower frequency ωi is called the idler. Because the OPO does not convert all the input energy into the signal and idler, a residual pump wave is also output.
OPOs need an optical resonator, but in contrast to lasers, OPOs are based on direct frequency conversion in a nonlinear crystal rather than from stimulated emission. OPOs exhibit a power threshold for an input light source (pump), below which there is negligible output power in the signal and idler bands.
OPOs include an optical resonator (cavity) and a nonlinear optical crystal. The optical cavity is an arrangement of mirrors that forms a resonator for light waves. Light confined in the cavity is reflected multiple times resulting in a multi-pass through the nonlinear crystal. The optical cavity serves to resonate at least one of the signal and idler waves. In the nonlinear optical crystal, the pump, signal and idler beams overlap.
While conventional lasers produce limited fixed wavelengths, OPOs may be desirable because the signal and idler wavelengths, which are determined by the conservation of energy and momentum (via phase matching), can be varied in wide ranges. Thus, it is possible to access wavelengths, for example in the mid-infrared, far-infrared or terahertz spectral region, which may be difficult to obtain from a laser. In addition, OPOs allow for wide wavelength tunability, for example, by changing the phase-matching condition. This makes OPOs a useful tool, for example, for laser spectroscopy. Utilizing additional nonlinear processes can further extend the range of accessible wavelengths (e.g. near-infrared, visible and/or ultraviolet spectral regions).
In addition, while prior light sources such as spectrally filtered plasma sources and supercontinuum white light lasers are available, these light sources suffer from poor photon (energy) efficiency (typically a few mW output power per nm). On the other hand, OPO/non-linear optics (NLO) technology may offer significantly higher energy efficiency with more narrow band output powers of greater than 10 mW. Thus, while supercontinuum and plasma sources produce a broad spectrum from which (for many applications requiring narrower bandwidth) parts are cut off, OPOs are capable of producing a tunable comparatively narrow band output (so there is no waste of power by filtering out). Therefore, there is a need in the industry to address one or more of the above mentioned shortcomings.